Transconductance filters are often used in electronic circuits to perform various filtering functions. Typically, these filters incorporate a transconductance amplifier which converts a voltage input signal into a controlled current signal, where the controlled current signal produced is equal to the product of the transconductance gain (g.sub.m) of the amplifier and the voltage input signal applied to the amplifier. This controlled current signal is then applied to a capacitor to produce an output voltage. Since the voltage across this capacitor varies in accordance with the rate of the change of the current passing through the capacitor and the current passing through the capacitor varies in accordance with the voltage applied to the transconductance amplifier, a frequency dependent filter circuit is created.
The characteristics (e.g. maximum gain, unity gain frequency, etc.) of this transconductance filter circuit are tunable via a control signal which is applied to the transconductance amplifier. Various methods can be employed to generate this control signal. For example, a simple potentiometer can be used to adjust the amplitude of the analog control signal applied to this transconductance amplifier. Alternatively, a digital-to-analog converter (DAC) can be used to convert a digital control signal to an analog signal prior to applying it to the transconductance amplifier, thus allowing digital control of the transconductance filter characteristics. By varying the control signal applied to the transconductance amplifier, the maximum gain of a transconductance filter can be adjusted along the gain axis which, in turn, shifts the unity gain frequency along frequency axis.
There are certain known shortcomings associated with these transconductance filters, including the known instability of the amplifier's transconductance gain (g.sub.m). Since this transconductance gain varies in accordance with several variables, such as temperature variations, process variations (such as transistor doping levels) and production variations (such as transistor channel width, transistor channel length, etc.), a transconductance filter circuit is prone to uncontrollable filter characteristic shifts which severely effect the stability and overall usability of the filter.
Various attempts have been made to improve the stability of these transconductance filter circuits. Since the filter characteristics of the circuit can be adjusted by varying the control signal applied to the transconductance amplifier, the effects of temperature, process and product variations on transconductance gain can be offset (or nullified) by varying the control signal applied to the transconductance amplifier. Therefore, it is possible to offset these filter drift characteristics by manually adjusting the control signal applied to the transconductance amplifier. However, manual adjustment is not always possible or practical.
Alternatively, an automatic transconductance filter controller circuit may incorporate a phase-lock loop (PLL) or delay-lock loop (DLL) to automatically adjust the control signal applied to the transconductance amplifier, thereby automatically offsetting the unwanted effects that temperature, process and product variations have on transconductance gain (g.sub.m) Specifically, by utilizing a PLL or DLL which incorporates one or more slave transconductance amplifiers within its loop, the output of this PLL or DLL is a signal which varies in accordance with the same variables (temperature, process and product) that affect the conductance gain (g.sub.m) of the transconductance filter circuit. Therefore, by providing this PLL or DLL generated signal to the control input terminal of the transconductance amplifier utilized in the transconductance filter circuit, the sporadic and uncontrollable drift characteristics of the filter circuit can be minimized.
However, there are several disadvantages associated with the use of this PLL or DLL as an automatic control circuit for a transconductance filter circuit. Specifically, these disadvantages include: the additional power required to drive such a PLL or DLL circuit; the additional wafer size (or PC board size) required to construct such a circuit; the complex design criteria and time required to construct such a PLL or DLL circuit; and the undesirable coupling of the PLL/DLL clock signal with the transconductance filter circuit which results in the imposition of PLL/DLL induced noise into the output of the transconductance filter circuit.